[LLVMdev] 'loop invariant code motion' and 'Reassociate Expression'
Preston Briggs
preston.briggs at gmail.com
Thu Apr 25 08:51:46 PDT 2013
It's an interesting problem.
The best stuff I've seen published is by Cooper, Eckhart, & Kennedy, in
PACT '08.
Cooper gives a nice intro in one of his lectures:
http://www.cs.rice.edu/~keith/512/2012/Lectures/26ReassocII-1up.pdf
I can't tell, quickly, what's going on in Reassociate;
as usual, the documentation resolutely avoids giving any credit for the
ideas.
Why is that?
Preston
> On Apr 23, 2013, at 10:37 AM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
>
> > As far as I can understand of the code, the Reassociate tries to
achieve this result by its "ranking" mechanism.
> >
> > If it dose not, it is not hard to achieve this result, just restructure
the expression in a way such that
> > the earlier definition of the sub-expression is permute earlier in the
resulting expr.
> >
> > e.g.
> > outer-loop1
> > x=
> > outer-loop2
> > y =
> >
> > inner-loop3
> > z =
> > t = z * y * z
> >
> > the RHS is better restructured into "x * y * z", such that x * y can
moved as early as possible.
> > Restructuring expr this expr is also able to reduce the critical path
even if no sub-expr is
> > moved out of loop.
> >
> > By no means can "canonicalization" can achieve this result, because
as this transformation need
> > some context. (It seems the name "cannonicalization" is badly abused in
LLVM; it could refers
> > to this kind of xform. IMO, cannonicalization is sort of blind but
consistent restructuring)
> >
> > BTW, I did similar stuff before in other compiler, I didn't see any %.
>
> Right. Reassociate ranks expressions by their order in the IR, so
shouldn't make matters worse, but it doesn't know about loops so can't
expose more of these opportunities AFAIK.
>
> Jeroen, it may be worth filing a bug. I'm not sure why LSR isn't handling
it later.
>
> For cases that LSR doesn't handle, we've been wanting an MI reassociate
pass in the backend anyway to expose ILP. That could clean up cases like
this when it's actually on the critical path and we don't mind the
regpressure.
>
> -Andy
>
> > On 4/23/13 6:48 AM, Jeroen Dobbelaere wrote:
> >> Hi,
> >>
> >> I am investigating a performance degradation between llvm-3.1 and
llvm-3.2
> >> (Note: current top-of-tree shows a similar degradation)
> >>
> >> One issue I see is the following:
> >> - 'loop invariant code motion' seems to be depending on the result of
the 'reassociate expression' pass:
> >>
> >> In the samples below I observer the following behavior:
> >>
> >> Both start with the same expression:
> >> %add = add i32 %total.0, %next.0
> >> %add1 = add i32 %add, 1
> >>
> >> -LLVM-3.1 converts this into:
> >> --after Reassociate expressions:
> >> %add = add i32 %next.0, 1
> >> %add1 = add i32 %add, %total.0
> >> -- after Canonicalize natural loops:
> >> %add = add i32 %next.0.ph, 1
> >> %add1 = add i32 %add, %total.0
> >> -- and during 'loop invariant code motion':
> >> LICM hoisting to while.cond.outer: %add = add i32 %next.0.ph, 1
> >>
> >> - LLVM-3.2 converts it into: (Notice the reversion of %total.0 and
%next.0)
> >> --after Reassociate expressions:
> >> %add = add i32 %total.0, 1
> >> %add1 = add i32 %add, %next.0
> >> -- after Canonicalize natural loops:
> >> %add = add i32 %total.0, 1
> >> %add1 = add i32 %add, %next.0.ph
> >> -- and during 'loop invariant code motion':
> >> -> %next.0.ph and '1' are loop invariant, but because those are
spreaded, they are not moved any more.
> >>
> >>
> >> Is there a way that 'Reassociate expressions' of 'LICM' can rearrange
the expressions (likely after 'Canonicalize natural loops'),
> >> and group loop-invariant parts of the expression ?
> >> Or is there another pass that I can/should enable to have this effect ?
> >>
> >> Greetings,
> >> Jeroen Dobbelaere
> >>
> >> ---
> >>
> >> llvm-3.1
> >>
> >>
> >> while.cond: ; preds = %sw.bb,
%sw.bb13, %sw.bb18, %sw.bb23, %if.end, %entry
> >> %next.0 = phi i32 [ 0, %entry ], [ %next.0, %if.end ], [ 8, %sw.bb23
], [ 8, %sw.bb18 ], [ 8, %sw.bb13 ], [ 4, %sw.bb ]
> >> %total.0 = phi i32 [ 0, %entry ], [ %total.1, %if.end ], [ %total.1,
%sw.bb23 ], [ %total.1, %sw.bb18 ], [ %total.1, %sw.bb13 ], [ %total.1, %
sw.bb ]
> >> %buf.0 = phi i8* [ null, %entry ], [ %buf.0, %if.end ], [ %4,
%sw.bb23 ], [ %3, %sw.bb18 ], [ %2, %sw.bb13 ], [ %1, %sw.bb ]
> >> %seed.addr.0 = phi i16 [ %seed, %entry ], [ %inc9, %if.end ], [
%inc9, %sw.bb23 ], [ %inc9, %sw.bb18 ], [ %inc9, %sw.bb13 ], [ %inc9, %sw.bb]
> >> %add = add i32 %total.0, %next.0
> >> %add1 = add i32 %add, 1
> >> %cmp = icmp ult i32 %add1, %dec
> >> br i1 %cmp, label %while.body, label %while.cond29
> >>
> >> *** IR Dump After Reassociate expressions ***
> >> define void @foo(i32 %size, i16 signext %seed, i8* nocapture %p)
nounwind {
> >> entry:
> >> %dec = add i32 %size, -1
> >> br label %while.cond
> >>
> >> while.cond: ; preds = %sw.bb,
%sw.bb13, %sw.bb18, %sw.bb23, %if.end, %entry
> >> %next.0 = phi i32 [ 0, %entry ], [ %next.0, %if.end ], [ 8, %sw.bb23
], [ 8, %sw.bb18 ], [ 8, %sw.bb13 ], [ 4, %sw.bb ]
> >> %total.0 = phi i32 [ 0, %entry ], [ %total.1, %if.end ], [ %total.1,
%sw.bb23 ], [ %total.1, %sw.bb18 ], [ %total.1, %sw.bb13 ], [ %total.1, %
sw.bb ]
> >> %buf.0 = phi i8* [ null, %entry ], [ %buf.0, %if.end ], [ %4,
%sw.bb23 ], [ %3, %sw.bb18 ], [ %2, %sw.bb13 ], [ %1, %sw.bb ]
> >> %seed.addr.0 = phi i16 [ %seed, %entry ], [ %inc9, %if.end ], [
%inc9, %sw.bb23 ], [ %inc9, %sw.bb18 ], [ %inc9, %sw.bb13 ], [ %inc9, %sw.bb]
> >> %add = add i32 %next.0, 1
> >> %add1 = add i32 %add, %total.0
> >> %cmp = icmp ult i32 %add1, %dec
> >> br i1 %cmp, label %while.body, label %while.cond29
> >>
> >>
> >> *** IR Dump After Canonicalize natural loops ***
> >> while.cond: ; preds =
%while.cond.outer, %if.end
> >> %total.0 = phi i32 [ %total.1, %if.end ], [ %total.0.ph,
%while.cond.outer ]
> >> %seed.addr.0 = phi i16 [ %inc9, %if.end ], [ %seed.addr.0.ph,
%while.cond.outer ]
> >> %add = add i32 %next.0.ph, 1
> >> %add1 = add i32 %add, %total.0
> >> %cmp = icmp ult i32 %add1, %dec
> >> br i1 %cmp, label %while.body, label %while.cond29.preheader
> >> [...]
> >>
> >> for.cond.for.end_crit_edge: ; preds = %for.body
> >> %split = phi i32 [ %inc, %for.body ]
> >> br label %for.end
> >> LICM hoisting to while.cond.outer: %add = add i32 %next.0.ph, 1
> >> LICM hoisting to while.cond.outer: %cmp2 = icmp eq i32 %next.0.ph, 0
> >> LICM hoisting to while.cond.outer: %cmp343 = icmp ult i32 0, %
next.0.ph
> >> LICM hoisting to while.cond.outer: %add740 = or i32 %next.0.ph, 1
> >> *** IR Dump After Loop Invariant Code Motion ***
> >> while.cond: ; preds =
%while.cond.outer, %if.end
> >> %total.0 = phi i32 [ %total.1, %if.end ], [ %total.0.ph,
%while.cond.outer ]
> >> %seed.addr.0 = phi i16 [ %inc9, %if.end ], [ %seed.addr.0.ph,
%while.cond.outer ]
> >> %add1 = add i32 %add, %total.0
> >> %cmp = icmp ult i32 %add1, %dec
> >> br i1 %cmp, label %while.body, label %while.cond29.preheader
> >> [...]
> >>
> >> -------
> >>
> >> llvm-3.2
> >>
> >>
> >> while.cond: ; preds = %sw.bb,
%sw.bb13, %sw.bb18, %sw.bb23, %if.end, %entry
> >> %seed.addr.0 = phi i16 [ %seed, %entry ], [ %inc9, %if.end ], [
%inc9, %sw.bb23 ], [ %inc9, %sw.bb18 ], [ %inc9, %sw.bb13 ], [ %inc9, %sw.bb]
> >> %total.0 = phi i32 [ 0, %entry ], [ %total.1, %if.end ], [ %total.1,
%sw.bb23 ], [ %total.1, %sw.bb18 ], [ %total.1, %sw.bb13 ], [ %total.1, %
sw.bb ]
> >> %next.0 = phi i32 [ 0, %entry ], [ %next.0, %if.end ], [ 8, %sw.bb23
], [ 8, %sw.bb18 ], [ 8, %sw.bb13 ], [ 4, %sw.bb ]
> >> %buf.0 = phi i8* [ null, %entry ], [ %buf.0, %if.end ], [ %4,
%sw.bb23 ], [ %3, %sw.bb18 ], [ %2, %sw.bb13 ], [ %1, %sw.bb ]
> >> %add = add i32 %total.0, %next.0
> >> %add1 = add i32 %add, 1
> >> %cmp = icmp ult i32 %add1, %dec
> >> br i1 %cmp, label %while.body, label %while.cond29
> >>
> >> *** IR Dump After Reassociate expressions ***
> >> define void @foo(i32 %size, i16 signext %seed, i8* nocapture %p)
nounwind {
> >> entry:
> >> %dec = add i32 %size, -1
> >> br label %while.cond
> >>
> >> while.cond: ; preds = %sw.bb,
%sw.bb13, %sw.bb18, %sw.bb23, %if.end, %entry
> >> %seed.addr.0 = phi i16 [ %seed, %entry ], [ %inc9, %if.end ], [
%inc9, %sw.bb23 ], [ %inc9, %sw.bb18 ], [ %inc9, %sw.bb13 ], [ %inc9, %sw.bb]
> >> %total.0 = phi i32 [ 0, %entry ], [ %total.1, %if.end ], [ %total.1,
%sw.bb23 ], [ %total.1, %sw.bb18 ], [ %total.1, %sw.bb13 ], [ %total.1, %
sw.bb ]
> >> %next.0 = phi i32 [ 0, %entry ], [ %next.0, %if.end ], [ 8, %sw.bb23
], [ 8, %sw.bb18 ], [ 8, %sw.bb13 ], [ 4, %sw.bb ]
> >> %buf.0 = phi i8* [ null, %entry ], [ %buf.0, %if.end ], [ %4,
%sw.bb23 ], [ %3, %sw.bb18 ], [ %2, %sw.bb13 ], [ %1, %sw.bb ]
> >> %add = add i32 %total.0, 1
> >> %add1 = add i32 %add, %next.0
> >> %cmp = icmp ult i32 %add1, %dec
> >> br i1 %cmp, label %while.body, label %while.cond29
> >>
> >>
> >> *** IR Dump After Canonicalize natural loops ***
> >> while.cond: ; preds =
%while.cond.outer, %if.end
> >> %seed.addr.0 = phi i16 [ %inc9, %if.end ], [ %seed.addr.0.ph,
%while.cond.outer ]
> >> %total.0 = phi i32 [ %total.1, %if.end ], [ %total.0.ph,
%while.cond.outer ]
> >> %add = add i32 %total.0, 1
> >> %add1 = add i32 %add, %next.0.ph
> >> %cmp = icmp ult i32 %add1, %dec
> >> br i1 %cmp, label %while.body, label %while.cond29.preheader
> >> [.....]
> >> for.cond.for.end_crit_edge: ; preds = %for.body
> >> %split = phi i32 [ %inc, %for.body ]
> >> br label %for.end
> >> LICM hoisting to while.cond.outer: %cmp2 = icmp eq i32 %next.0.ph, 0
> >> LICM hoisting to while.cond.outer: %cmp366 = icmp ult i32 0, %
next.0.ph
> >> LICM hoisting to while.cond.outer: %add763 = or i32 %next.0.ph, 1
> >> *** IR Dump After Loop Invariant Code Motion ***
> >> while.cond: ; preds =
%while.cond.outer, %if.end
> >> %seed.addr.0 = phi i16 [ %inc9, %if.end ], [ %seed.addr.0.ph,
%while.cond.outer ]
> >> %total.0 = phi i32 [ %total.1, %if.end ], [ %total.0.ph,
%while.cond.outer ]
> >> %add = add i32 %total.0, 1
> >> %add1 = add i32 %add, %next.0.ph
> >> %cmp = icmp ult i32 %add1, %dec
> >> br i1 %cmp, label %while.body, label %while.cond29.preheader
> >>
> >>
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> > __
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